Standards have been established for the exchange of electrical signals among processing devices. Processing devices include integrated circuit systems built on and using printed circuit boards by an increasingly wide array of suppliers. Architecture standards ensure that the various devices will, in fact, be able to communicate with one another as well as with central processing units that control the operation of such devices. These devices include high speed communication devices.
A channel is a pathway, e.g., physical, optical, etc., between the transmitters/receivers of individual devices such as cards, central processor, memory, etc., of a data transmission system as well as external interfaces. It is an important goal to provide a signal exchange between systems with little or no disruption.
Increasingly, an important feature of a communication channel is to provide for the transfer of greater quantities of signals (bandwidth) at faster propagation rates (high speed). Unfortunately, physical limitations, impedance, jitter, crosstalk, etc. associated with the physical interconnections and signal drivers and receivers can restrict high bandwidth, high speed signal transfer.
As transmission speeds move to 10 Gbps and beyond, designers must consider individual performance capabilities of components as well as the capabilities they exhibit when implemented in a system architecture. The interaction between components and channel results in unpredictable losses. This unpredictable nature limits the effectiveness of better materials and devices intended to compensate for predictable losses.
Many popular forms of encoding of the serial data result in signal frequencies in excess of 1 GHz. At such high frequencies, ISI (intersymbol interference) is often a problem. Intersymbol interference in a digital transmission system is the distortion of the received signal, which distortion is manifested in the temporal spreading and consequent overlap of individual pulses to the degree that the receiver cannot reliably distinguish between changes of state, i.e., between individual signal elements. At a certain threshold intersymbol interference will compromise the integrity of the received data. Intersymbol interference may be measured by eye patterns.
The integrity of the received data may be restored through equalization. Equalization may be used to correct the overlap of individual pulses, such that a receiver can again distinguish between individual signal elements. The equalized signal may then be subsequently processed with far less chance of error.
In digital communications an “eye diagram” is used to visualize how the waveforms used to send multiple bits of data can potentially lead to errors in the interpretation of those bits. Eye measurements are often used as an indication of the quality of the received and/or equalized signal. However, channel impairments tend to close the eye. More specifically, distortion of a signal waveform due to intersymbol interference and noise appears as closure of the eye pattern. A closed eye is indicative of a signal that cannot be received without an excessively high error rate. Thus, providing equalization to open the eye is essential. The sources of eye closure include jitter in the transmit and receive devices, ISI, and crosstalk. The statistical eye is a mathematical construct similar to an eye diagram, in which the statistical nature of these sources of eye closure are taken into account.
The statistical eye is an algorithm that effectively shows the probability of a given eye opening, given measurements of channel S-parameters, crosstalk S-parameters, and device (transmit and receive) jitter. Or, if a given eye opening at a given probability level is specified, the statistical eye becomes a tool to verify channel compliance. The original statistical eye algorithm incorporates two forms of equalization: feed-forward and decision-feedback. It is capable of determining the best equalizer of a given size or the best equalizer subject to a set of constraints. Channel compliance can then be verified assuming that interoperating devices will use equalizers of a given complexity.
Some communicating devices may implement Continuous-Time Equalization. This is an analog filter that shapes the frequency response—usually to invert the frequency response magnitude of the channel. Given that such devices exist, it can be seen that there is a need for a method and apparatus that incorporates Continuous-Time Equalization into the statistical eye channel compliance.